This invention relates to the field of deep-draft semi-submersible offshore platforms for the drilling of oil wells and natural gas wells and the production of oil and or gas from such wells. Specifically, the present invention relates to a type of deep-draft semi-submersible platform known as an Extendable Draft Platform, or “EDP.” More particularly, the present invention relates to an EDP with improved buoyancy columns that provide increased resistance to vortex-induced vibration (VIV).
One type of offshore platform that has met with commercial success in deep water applications is the semi-submersible platform. Conventional semi-submersible platforms, however, are subject to motions that make it difficult, or even impossible, to support the various types of risers that are employed in such platforms. Deep draft semi-submersible platforms have been proposed that would exhibit superior motion characteristics. One type of deep draft semi-submersible platform is known as the Extendable Draft Platform, or “EDP.” The typical EDP comprises a buoyant equipment deck having a plurality of openings (“column wells”) through the deck. The deck may conveniently be rectangular or triangular, with a column well at each comer or apex, although other configurations may be used. Installed in each of the column wells is a buoyancy column that can be ballasted (e.g., with seawater). The columns are initially installed in a raised position, and then lowered to a submerged position when the EDP has been moved to a deep water site. Each column is divided by transverse internal bulkheads and horizontal flats (decks) into a plurality of compartments, the compartments including means for introducing water into them for ballasting purposes when the columns are lowered to their submerged positions under a controlled procedure. Attached to the bottom of the columns is a heave plate assembly that helps to stabilize the EDP against the heave response of waves and swells. Examples of prior art EDPs are disclosed in U.S. Pat. No. 6,718,901-Abbott et al. and U.S. Pat. No. 6,024,040-Thomas, the disclosures of which are incorporated herein by reference.
In many regions intended for deployment of EDPs, strong sub-surface currents can cause vortex-induced vibration (VIV) to submerged structures, particularly elongate columns and the like. Prolonged exposure to VIV can result in structural failure due to fatigue damage to the components that are subject to the stresses caused by these motions. It is known that the provision of apparatus on elongated submerged hull structures for vortex breaking, or controlled vortex-shedding, can reduce or eliminate this problem. For example, in U.S. Pat. No. 6,148,751 and U.S. Pat. No. 6,349,664, there is described a hydrodynamic system for reducing vibration and drag on an elongated submerged hull. U.S. Pat. No. 6,244,785 describes elongated helical “strakes” disposed on a pre-cast concrete spar hull. U.S. Pat. No. 6,817,309, the disclosure of which is incorporated herein by reference, describes a spar-type offshore platform that employs helical strakes on submerged tubular cells that form a hull extending downwardly from the deck.
Heretofore, no practical way has been found to address the suppression of VIV in extendable draft platforms. While it would seem that adding strakes to the buoyancy columns of the EDP would be a solution, there are practical problems with this approach. For example, adding strakes to the buoyancy columns would increase the diameter of the columns, thereby increasing the diameter of the column wells through the deck needed to accommodate the columns. This would, in turn, create such problems as reducing the usable area of the deck. Alternatively, the overall diameter of the columns, including the strakes, can be maintained the same as the columns without the strakes. In other words, the tubular body of the column can be reduced by an amount equal to the width of the strakes. This approach, however, would reduce the effective water plane area of the columns, thereby degrading the overall stability of the EDP. Furthermore, there would be a need to protect the strakes from damage as the columns are lowered to their submerged positions through the column wells of the deck. Furthermore, there has been no practical way to fit the strakes onto the columns until after the columns are ballasted down. Thus, installation of the strakes would need to be performed as an underwater construction activity after the column lowering operation, thereby incurring substantial added costs. These considerations have left the EDP with no practical solution to the VIV problem.